As our society and the world continue to immerse itself in the technology and information revolution, our reliance on the electronic machines and computers that enable this technology continues to increase. From laptop PCs to complex mainframes, from fax machines to global telecommunications networks, from our personal FAX to the corner ATM, from e-mail to the Internet, from life saving and life sustaining medical equipment to computer controlled space exploration, and from personal convenience to national security, we are inextricably linked. Our lives, in many ways, exist within the critical data and processes computers and databases too numerous to count.
Along with the increasing reliance on computers and other sensitive electronic equipment comes the need to ensure that these machines remain reliably operative 24 hours a day, 365 days a year. While various methods exist to ensure data fidelity and machine operability and redundancy, it is clear that without electrical power the current revolution would quickly come to a standstill. Indeed, even the smallest disruption in electrical power can cause damage to a computer, a network, or other sensitive electronic equipment such as point-of-sale and process control equipment. At the very least, power problems can cause unexpected shutdowns and damage equipment. At the worst, bad power quality can cause data loss or corruption, or even destroy equipment. Even minor power problems can cost a company money. Any time a power interruption delays work in progress; valuable time is lost, and lost time means lost money.
While many are aware of the obvious problem of electrical power outages or blackouts that are usually caused by faults on the utility power system, there are many other power problems that can corrupt or destroy critical data, and damage equipment. Transients or power spikes, for example, caused by lightning or the switching of electrical loads can also destroy electronic circuitry and corrupt stored data. Likewise, power surges and overvoltages commonly caused by large electrical load changes and from utility power line switching can seriously damage electrical equipment. Further, power sags and brownouts that commonly occur when motors are started or as a result of a lack of capacity or faults in the power system can also cause sudden shutdowns in computer or process control equipment.
In recognition of these problems, uninterruptable power supplies (UPS) have been developed to provide, as their name implies, an uninterruptable and more robust (less prone to power problems described above) source of electrical power to sensitive electronic equipment. These UPSs are interposed between the sensitive electronic equipment and the electric utility input. They typically include a battery and power inverter that is capable of supplying AC power to the electronic equipment in the event of power loss or loss of power quality at the utility input. The power inverters may take many forms, but typically employ a sophisticated pulse-width modulated (PWM) scheme to convert the DC power from the battery to AC power for use by the electronic equipment. To avoid the requirement of large or multiple series connected batteries, linear transformers are typically used to step-up the inverter output AC voltage.
Unfortunately, the control algorithms required by these inverters to turn the power switches (typically insulated gate bipolar transistors (IGBTs)) on and off to construct a clean AC waveform are quite complex and require substantial computing power. The switching angle control is quite sensitive to variations in the attached load and requires highly accurate feedback circuitry to monitor the output voltage. Since the inverter is isolated from the output load by the linear transformer, a feedback signal transformer and other external circuitry are typically employed to provide the output voltage information to the controller for use in the control algorithms. This substantially increases the cost and complexity of the UPS system.
Depending on the sensitivity of the equipment to be powered, the UPS may allow the utility to supply power during normal operation, and may only switch to battery/inverter power during times of power outages or poor power quality. However, because utility power may experience sudden power problems as described above, most UPSs also include filtering to ensure that transients and other power glitches are not passed through to the equipment. In other words, additional circuitry is needed to provide output voltage regulation. This also increases the cost and complexity of the UPS system.
As an alternative to using a linear transformer and external regulation circuitry, the UPS described in U.S. Pat. No. 4,692,854, entitled Method and Apparatus for Modulating Inverter Pulse Width, issued to Baxter, Jr. et al., employs the use of a non-linear device known as a ferroresonant transformer. With its inherent regulation capability, this ferroresonant transformer is used to couple both the input AC voltage from the utility and the generated AC voltage from the inverter to the loads. While this design provides excellent performance, the control algorithms necessary for controlling the inverter to operate just outside of input saturation of the transformer are complex. These complexities are necessary in this prior design to avoid allowing the transformer to reach full core saturation. If the ferroresonant transformer were to reach full core saturation, the current would rise dramatically, would possibly cause component damage, and would certainly increase power dissipation.
As will be recognized by one skilled in the art, when a transformer saturates, the current it draws will ramp up after being stable for a period of time. Traditional saturation regulation depends on the detection of this ramp to shut off the switching devices. One of the disadvantages of this method is that any power put into the transformer after the saturation point is either dissipated as heat in any external snubbing circuitry (push-pull) or is dumped back into the battery (H-bridge). In either case, this energy is not available to the load during this time. Lastly, this method makes the power devices switch the resulting high current during saturation, resulting in higher switching losses. Both of these factors add up to less efficiency in the inverter.
As an alternative to saturation regulation that alleviates some of its drawbacks, regulation via feedback from the output voltage has also been attempted. Algorithms using output voltage feedback generally calculate an error between the present output voltage and the desired output voltage. This difference is then used to adjust the pulse width of the inverter accordingly. However, a signal transformer and other external circuitry are needed for this method because the output of the transformer is isolated from the inverter side of the system. This increases the complexity and expense of the system.
In addition to the complexities of current PWM algorithms, the physical size of the ferroresonant transformers used in these conventional systems is quite large and heavy. This size problem is driven mainly by the input voltage range from a typical utility, which can swing from 87 Vrms to nearly 148 Vrms for a standard 120 Vrms utility. To accommodate this voltage swing, the typical ferroresonant transformer requires that the input winding for the utility input use relatively large wires. This additional copper (larger wires) is necessary to handle the increased current draw through the primary that is required to supply a relatively constant output power to the load without overheating. That is to say, as the input voltage droops, the input current must increase to supply the same output power.
At the high voltage end of the spectrum, typical ferroresonant transformers require additional ferromagnetic material, typically steel, in the laminations to prevent saturation of the input core. As discussed above, saturation of the input core would result in a dramatic rise in input current and power dissipation, and could possibly damage the system. Such required robustness results in a typical ferroresonant transformer rated at approximately 3 kVA weighing approximately 75 pounds.
Therefore, there exists a need in the art for a new and improved UPS that overcomes these and other problems existing in the art. More specifically, there exists a need in the art for a new and improved UPS that does not require complex controls and sophisticated sensing circuitry for control of the PWM inverter during battery powered operation. Further, there exists a need for a UPS which includes a ferroresonant transformer that is lighter in weight, but that can still reliably operate over the utility input voltage range without overheating the input windings or saturating the input core.